Gradiometric DC superconducting quantum interference devices (SQUIDs) have been constructed from thin-film YBa2Cu3O7-xon MgO substrates. The SQUIDs were fabricated as forst-order gradiometers with PrBa2Cu3O7-xinsulator providing electrical isolation at the crossovers. Highly symmetrical first-order gradiometer flux transformers have also been fabricated. These couple to the SQUIDs and enhance the sensitivity to magnetic-field gradients. Each flux transformer has two 20 turn flat spiral SQUID input coils and a pair of larger-area balanced pickup coils. Flux transformers with both parallel and series configured pickup loops have been fabricated. When coupled to a SQUID as a flip chip, one gradiometer was operated at 77 K with a gradient sensitivity of (0.79±0.03) nT/m/ √Hz at 10 Hz and (2.0±0.1) nT/m/ √Hz at 1 Hz. © 1994.
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